1. Field of the Invention
The present invention generally relates to a fuzzy logic control apparatus and method, and more particularly, to an improved control method combining fuzzy logic control with sliding mode control, and also to application of the control method to a positioning mechanism which is a 2D servo positioning control device including a servo motor drive mechanism, a two-axis servo stage, a microcomputer control system, an interface convert control circuit, and an optical position detection mechanism. The positioning control method can not only be used with positioning control of 2D movement, but also be used in other fields such as positioning control of rotation or 3D movement.
2. Description of Related Art
With progress in semiconductor industry in recent years, various products tend to be light, thin and small, which imposes an increasingly higher precision requirement on fabrication thereof. Among others, precision positioning systems have been indispensable in semiconductor industry, optical communication industry, biomedical industry, MEMS, and the like. Traditionally used positioning systems include friction Piezo-actuated positioning stages, magnetic levitation positioning stages, friction drive positioning stages, lead screws positioning stages, and the like, each having its advantages and disadvantages. Usually, the lead screw positioning stage is widely welcomed in the industry because of its low price. However, the lead screw positioning stage can only reach micro-positioning and are unable to provide nano-positioning capability due to some of its mechanical characteristics, such as backlash problem. Therefore, conventional positioning stages with nano-positioning capability are all of Piezo-actuation, magnetic levitation or hydrostatics, which are expensive and have a short travel.
Currently, a most common method of traditional positioning system is to use a PID controller, which is advantageous in uncomplicated computation, but unable to meet high precision positioning control requirements. To address this problem, several intelligent control methods have been proposed for the positioning. One example of the methods is the use of a fuzzy logic controller, which uses a fuzzy logic rule [IF-THEN] to compute. However, this method requires a substantial computation resource and a long computation time, which makes the control device prohibitively expensive. Therefore, Taiwan Patent Number 300,290 proposed a defuzzification theory and design, providing a defuzzification method that does not need much data storage space and allows for relatively simpler computation. However, this Taiwan patent covers only improvements in the defuzzification part, and thus cannot achieve a higher speed of the overall computation.
U.S. Pat. No. 5,988,848 disclosed a novel control method which uses a hardware control circuit as a control parameter controller for a fuzzy sliding controller. Outputs of the fuzzy sliding controller and a plant are used as inputs of an adaptive controller, and an output of the adaptive controller is then used as parameters of the fuzzy sliding controller. As such, the output architecture of the fuzzy controller is variable. However, this controller needs to perform many complicated computation steps and, when used in practice, still consumes a lot of computation resources and time. As such, applying this fuzzy controller to precise positioning can be still difficult.
Having acknowledgement of the indispensability of the precision positioning stages and deficiency of various control methods, shortcomings need to be solved can be summarized as follows: (1) to reach high precision positioning, expensive positioning stages are required, making the positioning not cost-effective in practice; (2) if complicated control methods are used, then the computation time is increased and the hardware device is large in size, which is unpractical in use; and (3) the traditional control methods cannot achieve both quick transient response and steady state precise positioning. For these reasons, it is desired to improve the precision positioning stages and control method thereof.